1. Field of the Invention
This invention relates to an optical information detecting device, and, in particular, to an optical pick-up device suitable for use in an optical disc memory system.
2. Description of the Prior Art
An optical pick-up device for use in an optical disc memory system is well known in the art. Such an optical pick-up device is provided for storing or retrieving information on or from a recording disc. For this purpose, use is typically made of a laser light source which emits a laser beam which is caused to impinge on the recording disc through the optical pick-up device. Since there is a relative movement between the recording disc and the optical pick-up device, and the laser beam is required to be focused on the recording disc at all times, the optical pick-up must be capable of detecting a focusing error. Based on such a detected focusing error, the relative distance between the optical pick-up and the recording medium is adjusted so as to maintain the required focusing condition. A knife-edge scheme is typically used in such an optical pick-up device and a prior art pick-up device employing such a knife-edge scheme for detecting a focusing error is disclosed, for example, in Japanese Laid-Open Pub. No. 58-70434.
In detecting a focusing error in such an optical pick-up device, there are two key factors: high sensitivity and broad detecting range. That is, the higher the sensitivity in detecting a focusing error, the finer the focusing control, and the broader the detecting range, the more stable the focusing control. There has been a need to develop a novel optical pick-up capable of detecting a focusing error at high sensitivity over a broader detecting range.